1. Field of the Invention
The present invention relates to a method for signal processing generally, and more particularly, to a method for decoding multiple wireless signals in the presence of signal diversity.
2. Description of the Related Art
A. Coherent and Non-Coherent Modulation
Phase shift keying (PSK) and quadrature amplitude modulation (QAM) are the most widely used modulation schemes in digital communications. For both modulations, the phase of the signal carries the information that needs to be recovered at the receiver. Coherent modulation uses the absolute phase of the signal to represent the information whereas non-coherently modulated schemes, e.g., those implementing differential encoding, embeds the information in the phase difference between consecutive symbols. As such, for coherent modulation, one needs to keep track of the channel state, especially the channel phase information, whereas for differentially encoded signals, there is no such need so long as the channel coefficients do not vary much from time to time.
In certain applications, coherent modulation schemes are less favored because tracking channel state information puts an additional burden on the communication system, which is exacerbated with fast fading channels in a mobile environment. Thus in many practical systems such as satellite and radio relay communications, as well as in some cellular systems, non-coherent modulations are favored.
B. LDPC Codes
LDPC codes are a class of linear block codes with a particular characteristic in terms of their parity check matrix. Specifically, the fraction of nonzero entries is small, a property known as sparsity. LDPC codes provide a performance close to the Shannon limit for a number of important channels. In other words, one cannot expect to have codes that perform better than LDPC in terms of transmission rate and reliability tradeoff. Furthermore, the decoding algorithms have linear time complexity. These advantages, i.e., the superior error correction performance and simplicity in implementation makes it the most widely used error correction codes in existing and future wireless communications systems, including the digital television broadcast standard (DVB-S2), ITU-T G.hn standard. LDPC is also used for 10 GBase-T Ethernet, which sends data at 10 gigabits per second over twisted-pair cables. As of 2009, LDPC codes are also part of the Wi-Fi 802.11 standard as an optional part of 802.11n [4] and 802.11ac, in the High Throughput (HT) PHY specification.
C. Hard and Soft Decoding
The decoding algorithms for LDPC codes can be classified into two main categories: hard-decision decoding and soft-decision decoding. The difference between the two lies in the inputs that are taken in by the algorithm. For the hard-decision decoding, the inputs are decoded symbols from the demodulator, while likelihood ratio values are inputs for the soft-decision algorithms. In other words, the hard decision-decoding algorithm receives only that data which is received from the demodulator. If the data has been corrupted during transmission, or signal-loss has otherwise occurred, the corrupted information will be demodulated as-is and sent to the algorithm, yielding potentially poor decoding performance. Soft-decision decoding, based on the concept of belief propagation, will return a received value, along with a form of confidence metric so that the algorithm can make a better decision about the original information sent from the transmitter. As a result, soft-decision decoding yields a better decoding performance and is therefore the preferred method where possible.
In recent communication systems, multiple independently received signal copies of the same message are often available at the receiver—this is known as diversity reception. Diversity reception will occur in a variety of situations including, retransmission in packet based systems, multi-channel environments, and multi-antenna receivers that have become prevalent in almost all current and likely future wireless systems. Diversity reception provides the potential that the received signal be combined to recreate the original signal which often lead to significantly better reception compared with that of a single receive channel.
While non-coherent combining techniques have been studied for other channel codes, such as Trellis codes, and Turbo codes, there is a need for non-coherent combining techniques for LDPC coded signal. In particular, there is a need for a diversity combining scheme for non-coherent modulation with soft decisions to take advantage of the performance gain compared with the hard-decision combining scheme. Accordingly, there is a need in the art for a diversity combining scheme with soft decisions for LDPC coded signals.